Organic semiconductor based on phenylethynyl end-capped anthracene

Thin Solid Films 519 (2011) 7998–8002

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Organic semiconductor based on phenylethynyl end-capped anthracene Seul-Ong Kim a, Myung Won Lee b, Sang Hun Jang a, So Min Park a, Jong Won Park a, Moon-Hak Park a, So Hee Kang c, Yun-Hi Kim c,⁎, Chung Kun Song b,⁎, Soon-Ki Kwon a,⁎ a b c

School of Material Science & Engineering and ERI, Gyeongsang National University, Jinju 660-701, Republic of Korea Department of Electronics Engineering, Dong-A University, Busan, 604-714, Republic of Korea Department of Chemistry and RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 24 November 2010 Received in revised form 20 May 2011 Accepted 23 May 2011 Available online 30 May 2011 Keywords: Organic semiconductors Anthracene Rod structure Oxidation Stability Field effect mobility Reproducibility

a b s t r a c t Organic semiconductor based on anthracene with phenylethynyl end capping units, which has rigid rod-like structure with high aspect ratio, was synthesized by Sonogashira coupling reaction. The structure of obtained material was confirmed by fourier transform infrared spectroscopy, mass and elemental analysis and its physical properties were characterized by ultraviolet–visible spectroscopy and cyclovoltammetry. 2,6-Bisphenylethynyl-anthracene (BPEA) has deeper highest occupied molecular orbital level with higher stability as well as high intermolecular π–π stacking. The crystalline and morphological properties of BPEA thin film were studied using X-ray diffraction and atomic force microscopy. BPEA forms high long range ordered structure in the film and it exhibited high field effect mobility of 0.42 cm 2 V− 1 s− 1 (on/off ratio of 107) with device reproducibility and high oxidation stability. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic thin film transistors (OTFTs) based on molecular and polymeric organic semiconductors have attracted considerable interest due to their lower cost deposition processes and higher compatibility with plastic substrates compared to silicon-based thin film transistors (TFT) [1–3]. They also have diverse potential applications for integrated circuits of flexible display panels, new generation smart cards, integrated logic circuits, and sensors [4–7]. Many types of organic semiconductors, thiophene [8–10] and selenophene derivatives [11,12] and fused aromatic derivatives, such as pentacene [13,14], tetracene [15,16], anthracene [17–20], dithieno-thiophene [21,22], thienothiophene [23], have been reported for OTFT. These fused aromatic derivatives have rich π-electron density with a high aspect ratio, and symmetry, which lead to compact π-stacking and long range orderings [19,20]. Pentacene is well known OTFT material with a highest mobility with a high on–off ratio, a low threshold voltage (Vth) and low subthreshold swing (ss) [13,14]. However, pentacene has drawbacks, including low oxidative stability and formation of endo-peroxide [24,25]. Therefore, tetracene, anthracene and naphthalene derivatives with lower HOMO level and higher stability compared to pentacene have been reported as OTFT materials [15–20].

⁎ Corresponding authors. 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.060

Recently, our group reported dialkoxynaphthalene end-capped anthracene derivatives and 2-hexylthienothiophene end-capped anthracene derivatives. The anthracene derivatives with electron rich end groups showed good stability and mobility due to enhanced π–π stacking, resulting from its densely packed crystal structure [6,26]. Especially, dialkoxynaphthalene end-capped anthracene derivatives showed three times higher mobility (0.64 cm2/V·s) than that (0.2 cm2/ V·s) of pentacene under the same condition [26]. In this study, the rigid rod-like structured 2,6-bis(phenylethynyl) anthracene (BPEA) having anthracene core and phenylethynyl end groups is designed and synthesized. The designed BPEA was expected to have deep highest occupied molecular orbital (HOMO) level, leading to higher stability than that of pentacene. Moreover, rigid rod structure with a high aspect ratio due to triple bond, leads to a higher long range ordering and a higher intermolecular π–π stacking compared to that of pentacene. Thus, the device incorporating BPEA shows a high charge mobility, stability and device reproducibility.

2. Experimental details 2.1. Materials Phenylacetylene, trans-dichlorobis(triphenyl phosphine)palladium were purchased from Aldrich. All reagents purchased commercially were used without further purification. Tetrahydrofuran (THF) and diethylether were dried over sodium/benzophenone.

S.-O. Kim et al. / Thin Solid Films 519 (2011) 7998–8002 Table 1 Characteristic of OTETs based on BPEA and pentacene.

Pentacene BPEA BPEA (2 weeks)a

Mobility (cm2/V·S)

SS (V/dec)

On/off ratio

Vth (V)

0.21 0.42 0.30

2.43 1.12 1.83

4.82 × 105 1.25 × 107 1.46 × 106

7.4 1.1 1.9

a TFT device was studied by monitoring its TFT characteristics over time in air at 45% relative humidity. The mobility of the device degraded slightly after standing 2 weeks in air.

2.2. 2,6-Dibromo-anthraquinone (1) 2,6-dibromo-anthraquinone was synthesized using the procedure reported in the literature [27]. 2.3. 2,6-Dibromo-anthracene (2) A mixture of 2,6-dibromoanthraquinone (10 g, 54 mmol), acetic acid (100 mL), hydroiodic acid (75 mL) and hypophosphorous acid (40 mL) was refluxed at 150 °C for 4 days. The color was changed from green, orange to yellow. The precipitate was filtered and washed with ethanol. The pure product was obtained by extraction using soxhlet with toluene to pale yellow crystals. Yield: 5.4 g (50%), 1H-NMR (300 MHz, CDCl3): δ 8.31 (s, 1H), 8.18 (s, 1H), 7.87–7.90 (d,1H), 7.53– 7.56 (d, 1H). 2.4. 2,6-Bis-phenylethynyl-anthracene (BPEA) The general procedure for the Sonogashira coupling reaction was followed using 2 (3 g, 9 mmol), phenyl-acetylene (6.70 g, 66 mmol), copper(I) Iodide (0.312 g, 2 mmol), trans-dichlorobis(triphenyl phosphine)palladium(II) (1.151 g, 2 mmol), toluene (30 mL) and triethylamine (30 mL). The mixture was refluxed to 120 °C for 18 h over. After removing solvent, the precipitate was filtered and washed with dichloromethane. Purification by extraction using soxhlet with toluene gets pale green crystals. Yield: 2.41 g (71.32%), EI-MS: m/z = 378 (M+), FT-IR (KBr, cm− 1): 3047 (aromatic C-H), 2171 (C≡ C), Anal. Calcd for C30H18: C, 95.21%; H, 4.79%. Found: C, 94.97%; H, 4.83%. 2.5. TFT fabrication TFT devices were fabricated using the following structure. The OTFTs employed aluminum as gate electrodes which were evaporated and patterned by lift-off process. To prepare the organic gate insulator, we mixed poly(4-vinylphenol) (PVP) and a cross-linking agent (CLA: poly(melamine-co-formaldehyde)) with a propylene glycol monomethyl ether acetate solution. The mixed solution was O

stirred for 2 h with a magnetic stirrer and then filtered through a syringe filter with a 0.45 μm poly(tetrafluoroethylene) membrane. The PVP film was spin-coated on the substrate and then baked in a convection oven to activate the CLA. The baking temperature was about 200 °C for a duration of 20 min. The dielectric constant was 3.6 at 1 MHz and the capacitance density was 9.1 nF/cm 2 for the thickness of 350 nm. Subsequently, the source and drain electrodes were formed using Au which were evaporated and patterned by lift-off process. And then organic semiconductor (OSC) (pentacene or BPEA) layer was deposited on 80 °C heated substrate by thermal evaporation through a shadow mask, respectively. In our case, the optimum thickness was found to be 50 nm evaporated at 0.3 Å/s for both OSCs. The OTFTs of bottom contact structure were completed. The performance parameters, such as field effect mobility and threshold voltage, were extracted from the transfer curves by comparing it with the following equation in the saturation region (μsat = (2IDSL)/(WCi (Vg–Vth) 2)), and were summarized in Table 1, where IDS is the saturation drain current, W is the channel width, L is the channel length, μ is the field effect mobility, Ci is the capacitance of the PVP gate dielectric, Vg is the gate voltage, and Vth is the threshold voltage.

2.6. Measurements A Genesis II fourier transform infrared spectroscopy (FT-IR) spectrometer was used to record IR spectra. 1H-NMR(nuclear magnetic resonance) spectra was recorded with the use of Avance 300 MHz NMR Bruker spectrometers, and chemical shifts are reported in ppm units with tetramethylsilane as internal standard. Thermogravimetric analysis (TGA) was performed under nitrogen on a TA instrument 2050 thermogravimetric analyzer. The sample was heated using a 10 °C/min heating rate from 50 °C to 800 °C. Differential scanning calorimeter (DSC) was conducted under nitrogen on a TA instrument 2100 differential scanning calorimeter. The sample was heated with the 10 °C/min from 30 °C to 300 °C. Mass spectrum was measured by Jeol JMS-700 mass spectrometer. UV–vis absorption spectra and photoluminescence (PL) spectra were measured by Perkin Elmer LAMBDA900 UV/VIS/NIR spectrophotometer and LS-50B luminescence spectrophotometer, respectively. Cyclic voltammograms of BPEA were recorded on an epsilon E3 at room temperature in a 0.1 M solution of tetrabutylammonium perchlorate (Bu4NClO4) in acetonitrile under nitrogen gas protection at a scan rate of 50 mV/s. A Pt wire was used as the counter electrode and a Ag/AgNO3 electrode as the reference electrode. atomic force microscopy (AFM) (Multimode IIIa, Digital Instruments) operating in tapping-mode was used to image surface morphology. Synchrotron X-ray diffraction analysis for semiconductor film was performed at 10 C1 beam line at Pohang Accelerator Laboratory (PAL). O

NH2 t-BuNo,CuBr 2 CH3CN

H2N

Br

Br

HI,H3PO2 AcOH

Br

O

Br

O 2

1

2

7999

+

H

Pd(pph3)2Cl2,CuI NEt3,toluene

BPEA

Scheme 1. Synthetic scheme of BPEA.

8000

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1.2 UV-Solution UV-Film PL-Solution PL-Film

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Wavelength (nm) Fig. 1. UV and PL spectra of BPEA in toluene solution and film.

The film microstructure and morphology of a vapor-deposited thin film of BPEA grown on cross-linked PVP were investigated by X-ray diffraction and AFM. The X-ray diffraction spectra and atomic force microscopy morphology of a BPEA thin film vacuum deposited at 80 °C were investigated.

3. Results and discussion The target molecule, 2,6-bis-phenylethynyl-anthracene (BPEA) was prepared using a well-known reaction. Bromination of diaminoanthraquionone and the reduction of dibromoanthraquinone to dibromoanthracene were carried out via procedures as reported elsewhere [17,28]. As shown in Scheme 1, BPEA was synthesized by Sonogashira reaction as shown in the Scheme 1. The reaction was carried out in co-solution of toluene and triethylamine (1:1). The obtained BPEA was purified by extraction using soxhlet with toluene, recrystallization and subsequent sublimation. The compound was confirmed using (FT-IR) (S1), high-resolution mass spectrum and elemental analysis. The photophysical properties of the BPEA were studied using UV– visible absorption and PL spectra in toluene solution and film, respectively, as shown in Fig. 1. The maximum absorption of BPEA in film was red-shifted by 30 nm when compared to that in the solution, indicating the strong intermolecular interactions in the thin film. In addition, the maximum PL emission of BPEA film was 491 nm red-shifted when compared to that (423 nm and 440 nm) of the solution state. The optical band gap of BPEA was determined to be 2.63 eV from the onset absorption edge at 471 nm. The value is much wider than that of pentacene (702 nm, 1.77 eV).

Fig. 3. Cyclic voltammogram for BPEA in 0.1 M tetrabutylammonium perchlorate solution of acetonitrile.

The thermal property of BPEA was evaluated by TGA in nitrogen atmosphere. TGA result revealed that BPEA has good thermal stability up to 305 °C (Fig. 2). Cyclic-voltammetry measurement was carried out in acetonitrile using a BPEA coated carbon electrode as the result shown in Fig. 3. It showed an irreversible oxidation peak potential (Eonset) at 1.40 V, indicating good stability. The ionization potential (IP) of BPEA calculated using ferrocene as a standard was 5.84 eV, which was deeper than that of both pentacene and dialkoxynaphthalene end-capped anthracene. From these results, it is expected that BPEA will have good stability, which is a major concern with pentacene. Fig. 4 shows the X-ray diffractogram of BPEA film deposited on top of glass/cross-linked polyvinylphenol (PVP) substrates at 80 °C. The film revealed narrow intense peaks with multiple orders of reflection. The peak at 2θ = 3.90° is assigned to the first-order reflection and the remaining peaks are to successive orders of reflections. The primary peak showed strong diffraction with a d-spacing value of 22.62 Å, which was very close to the BPEA length 22.78 Å as calculated by Hyper Chem 7.0. This indicates that the BPEA molecules were aligned nearly perpendicular to the substrate and formed perfectly packed structure. Fig. 5 shows AFM images in which very large and connective grains with terrace-like step structures were obtained. This may have resulted from the substantial increase in size of the grains, in which the molecules are closely packed parallel to each other in vertical arrangement, allowing faster growth in directions parallel to the layers than perpendicular to them. In this way, the crystals form thin plates with a terraced structure as the molecules form multiple layers [10]. The grain size was greater than 0.5 μm and the step height between layers was 22.63 Å. The step height is in good agreement with the value of the interlayer spacing as deduced from X-ray

100 001

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Weight loss(%)

600

90

85

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Fig. 2. TGA curve of BPEA.

600

700

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2 Theta(° ) Fig. 4. X-ray diffraction (XRD) patterns of BPEA deposited on cross-linked PVP at 80 °C.

S.-O. Kim et al. / Thin Solid Films 519 (2011) 7998–8002

8001

Table 2 Average and standard deviation of OTFTs on BPEA and pentacene. BPEA

Mobility (cm2/V·S) SS (V/dec) On/off ratio VT (V) Off-state current (pA/um)

Fig. 5. AFM images of BPEA deposited on cross-linked PVP.

analysis, confirming the existence of single-molecule-thick layers that have an edge-on orientation. The device was fabricated bottom contact structure using BPEA for the active layer, cross-linked PVP for the gate dielectric, and Au for the source/drain electrodes with a channel length of 30 μm and a width of 150 μm. To make a comparison of the device characteristics between pentacene and BPEA, the devices were made under the same conditions

a

-7.0µ VG=-30

-6.0µ

IDS [A]

-5.0µ

VG=-25

-4.0µ -3.0µ -2.0µ -1.0µ µ 0.0 0

-5

-10

-15

-20

VDS[V]

Pentacene

Average

Standard deviation

Average

Standard deviation

0.40 0.95 8.93 × 106 1.48 0.0094

0.011 0.18 5.71 × 106 1.85 0.0147

0.213 2.433 4.8 × 105 7.42 0.056

0.0207 0.433 1.36 × 105 0.592 0.0142

which is the optimized for pentacene. The devices showed typical p-channel FET properties under ambient conditions. Fig. 6 shows the typical transfer and output characteristics of BPEA at 80 °C. The field effect mobility (μ) was 0.42 cm2/V·s, which is two times higher than pentacene. Other performance parameters were extracted from the transfer characteristic curve, revealing an on/off ratio of 107, Vth as low as 1.1 V, and ss of 1.12 V/dec. These values were also comparable to those of pentacene. Moreover, the mobility of good devices by using BPEA as semiconducting layer shows much lower standard deviation than pentacene (Table 2). It may be due to the good stability of BPEA. The ambient stability of an illustrative TFT device was studied by monitoring its TFT characteristics over time in air at a relative humidity of 45% (Fig. 7). The mobility of the device degraded slightly after standing for two weeks in air. The BPEA device retained a high mobility value of 0.3 cm2/V·s with an on/off ratio greater than 106. Pentacene TFTs have been reported to be degraded in air and under electrical stress as well [29–31]. We also measured the stability of pentacene TFTs (S2). In air the mobility quickly decreased for bottom contact structured (BCS) as well as for top contact structured OTFTs (TCS). The degradation was much more severe for BCS by exhibiting 40% reduction after the initial few hours. The major reason may be due to oxidation of pentacene molecules [32,33]. In summary, an organic material with high aspect ratio, which is combined structure of fused aromatic and phenylethynyl group, was synthesized by Sonogashira reaction and was characterized using electrochemistry, UV–vis absorption and photoluminescence and XRD studies. BPEA showed a high ionization potential, high intermolecular π-stacking and a well-ordered crystalline structure. This material exhibits high OTFT characteristics, low standard deviation and high stability superior to pentacene. Considering the improved performance of the recently reported pentacene based OTFT devices, the transistor performance of BPEA can be expected to be improved greatly by changing the gate insulators and controlling the grain sizes.

b 2.5m

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10-14 20

VGS [V] Fig. 6. a) Output characteristics and b) transfer characteristics of a BPEA OTFT devices (L = 30 μm, W = 150 μm) fabricated at Tsub 80 °C.

-4

Fig. 7. The characteristics of stability of OTFT device based on BPEA.

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Acknowledgment This work was supported by a grant (F0004011-2010-33) from the Information Display R&D Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy and Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education, Science and Technology (2011-0000310) and the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in Strategic Technology. References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] Z. Bao, Adv. Mater. 12 (2000) 227. [3] J.W. Park, D.H. Lee, D.S. Chung, D.M. Kang, Y.H. Kim, C.E. Park, S.K. Kwon, Macromolecules 43 (2010) 2118; . D.S. Chung, S.J. Lee, J.W. Park, D.B. Choi, D.H. Lee, J.W. Park, S.C. Shin, Y.H. Kim, S.K. Kwon, C.E. Park, Chem. Mater. 20 (2008) 3450; . Y.N. Li, T.H. Kim, Q.H. Zhao, E.K. Kim, S.H. Han, Y.H. Kim, J. Jang, S.K. Kwon, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 5115; . D.S. Chung, J.W. Park, S.O. Kim, K. Heo, C.E. Park, Y.H. Kim, S.K. Kwon, Chem. Mater. 21 (2009) 5499; . M.J. Lee, M.S. Kang, M.K. Shin, J.W. Park, D.S. Jung, C.E. Park, S.K. Kwon, Y.H. Kim, J. Polym. Sci. A Polym. Chem. 48 (2010) 3942; . J.U. Ju, D.S. Chung, S.O. Kim, S.O. Jung, C.E. Park, Y.H. Kim, S.K. Kwon, J. Polym. Sci. A Polym. Chem. 47 (2009) 1609; . T.T.M. Dang, S.J. Park, J.W. Park, D.S. Chung, C.E. Park, Y.H. Kim, S.-K. Kwon, J. Polym. Sci. A Polym. Chem. 45 (2007) 5277. [4] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [5] C. Reese, Z. Bao, Materialstoday 10 (2007) 20. [6] H.-S. Kim, Y.-H. Kim, T.-H. Kim, Y.-Y. Noh, S.M. Pyo, M.H. Yi, D.-Y. Kim, S.-K. Kwon, Chem. Mater. 19 (2007) 3561. [7] J.-H. Park, D.S. Chung, J.-W. Park, T. Ahn, H.Y. Kong, Y.K. Jung, J. Lee, M.H. Yi, C.E. Park, S.-K. Kwon, H.-K. Shim, Org. Lett. 9 (2007) 2573. [8] A. Dodabalapur, L. Torsi, H.E. Katz, Science 268 (1995) 270. [9] M. Melucci, M. Gazzano, G. Barbarella, M. Cavallini, F. Biscarini, P. Maccagnani, P.J. Ostoja, Am. Chem. Soc. 125 (2003) 10266. [10] H.K. Tian, J.W. Shi, D.H. Yan, L.X. Wang, Y.H. Geng, F.S. Wang, Adv. Mater. 18 (2006) 2149.

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